Comparing Water Potential of Two Plant Tissues

Comparing water potential of two plant tissues

Introduction

Osmosis is a passive transport and stands for diffusion of water across the plasma membrane, which is selectively permeable. Taking in account the difference in free water – water not bound to solutes or surfaces – concentration, H2O diffuses from the area of lower solute concentration to the area of higher solute concentration. When plant cells, which have walls, are placed in a hypotonic environment, the wall helps preserve the cell's water balance. Just as in animal cells, the water fills the cell and causes swelling, but the inelasticity of the cell wall prevents it from bursting by applying back pressure that prevents further water intake. In hypotonic environment, cells become turgid, which is the optimal state for most plant cells. In isotonic surroundings, plant cells are flaccid. Nevertheless, in the case of hypertonic environment, a plant cell acts the same as an animal cell; plasma membrane separates from the cell wall, causing the cell to lose water to the environment and shrink. This process is called plasmolysis.

Water potential quantifies the affinity of water to move from a region of higher water potential to a region of lower water potential due to osmosis, taking in account solute concentration and physical pressure of the cell wall. It measures the potential energy of water per unit volume relative to pure water. It is frequently represented by the Greek letter ?, standing for potential energy per unit volume. It is measured in units of pressure called megapascals, where pure water in normal circumstances equals 0 MPa. The pressure inside of a plant cell is circa 0.5 MPa. The water potential equation is: ? = ?s + ?p, showing that both solute concentration (?s) and pressure (?p) affect water potential. Solute potential or osmotic potential, as it is also called since solutes affect the direction of osmosis, is proportional to a solutions molarity. When solutes are added, they bind water molecules, lowering the number of free water molecules and reducing the capacity of the water to move and do work, thus decreasing water potential. Pressure potential on the other hand, depends on atmospheric pressure, hence making it possible for it to be either negative or positive.

Research question

The research question of this investigation was, how in terms of water potential, tissues are affected when placed in different solutions.

Hypothesis and null hypothesis

With increasing concentration of sucrose solution, the mass of both tissues will decrease. The null hypothesis is that there is no significant difference in the affection of the tissue of a potato and the tissue of an apple.

Variables

The independent variable is the concentration of the sucrose solution and the dependant variable is the volume of the potato and apple tissue. Controlled variable is the amount of the sucrose solution and the temperature of the environment in which we performed the experiment.

Method and procedure

Material and procedure was provided by the teacher.

Data collection and processing

Quantitative raw data

Table 1. Weight of potato tissue before exposure to the sucrose solution and after exposure to sucrose solutions of different concentrations. The uncertainty of measurement is ± 0.01 g

Sucrose concentration

(mol/

dm3) Weight of sample before exposure(g)

± 0.01 g Weight of sample after exposure

(g)

± 0.01 g

0.0 4.87 5.66

4.69 5.29

5.28 6.05

4.68 5.40

4.91 5.65

0.2

5.17 5.40

4.84 5.17

4.97 4.99

5.14 5.40

5.17 5.42

0.4

5.02 4.58

5.03 4.61

5.18 4.84

4.78 4.35

4.94 4.36

0.6

4.70 3.35

4.95 3.76

4.80 3.41

4.99 3.94

5.09 3.83

0.8

4.88 3.45

5.18 3.50

5.08 3.62

4.90 3.37

4.96 3.49

1.0

5.06 3.45

5.24 3.50

4.97 3.62

4.88 3.37

4.99 3.49

Table 2. Weight of apple tissue before exposure to the sucrose solution and after exposure to sucrose solutions of different concentrations. The uncertainty of measurement is ± 0.01 g. The data were acquired by a colleague.

Sucrose concentration

(mol/

dm3) Weight of sample before exposure(g)

± 0.01 g Weight of sample after exposure

(g)

± 0.01 g

0.0 3.62 4.00

3.66 4.03

3.69 4.05

3.85 4.15

3.71 3.94

0.2

4.11 5.12

4.25 4.89

3.90 4.60

3.96 4.58

3.94 4.44

0.4

3.89 4.46

3.65 4.55

4.18 4.92

3.92 4.70

3.64 4.34

0.6

3.73 4.18

3.56 3.84

3.76 4.34

3.59 4.19

4.00 4.38

0.8

4.11 4.38

3.75 3.87

3.75 3.91

3.80 3.79

3.84 3.93

1.0

3.20 2.98

3.57 3.37

4.29 4.05

4.01 3.84

3.80 3.60

?

Qualitative raw data

Sketch 1. Effect of distilled water – hypotonic environment – on onion cells.

Sketch 2. Effect of 1 mol dm-3 sucrose solution – hypotonic environment – on onion cells.?

Processed quantitative data

Table 3. Difference in weight between unexposed potato tissue and potato tissue exposed to different sucrose concentrations, the average difference, percentage difference and average percentage difference.

Sucrose concentration

(mol/

dm3) Difference in weight between unexposed and exposed sample

(g) ±0.01g Average difference

(g)

±0.02g Percentage difference

(%)

±0.02g Average percentage difference

(%)

±0.02g

0.0 0.79 0.72 16.22 14.81

0.60 12.79

0.77 14.58

0.72 15.38

0.74 15.07

0.2

0.23 0.26 4.45 5.24

0.33 6.82

0.25 5.03

0.26 5.06

0.25 4.84

0.4

-0.44 -0.44 -8.76 -9.12

-0.42 -8.35

-0.34 -6.56